The invention relates to a mirror element for the reflection of x-rays particularly in EUVL-exposure systems for the manufacture of semiconductors, wherein the x-rays reach the mirror element under a large incident angle. The mirror element comprises essentially a substrate on which multiple layers of, alternately, metal and non-metal materials are disposed.
Based on statistical estimates semi-conductor structures become smaller every 2-3 years by a factor ½, whereby the density of the transistors on the semiconductor structures are doubled in this time frame. It is therefore envisioned as a target for that industry that in the year 2005 semiconductor structures will become smaller than 100 nm, see for example National Technology Roadmap for Semiconductors in “Soft x-ray and extreme ultraviolet radiation”, David Attwood, Cambridge University Press, 1999. In order to be still able to manufacture such small structures by lithographic exposure procedures, visible light is unsuitable as the light source. Instead, for future exposure procedures the use of light of shorter wavelength will be necessary for example, in the form of “Extreme Ultraviolet Radiation (EUV) or soft x-rays. Since the optical properties of materials are highly dependent on the wavelength and those materials which reflect light in the visible range will generally not reflect light in the EUV range or in the range of soft x-rays, conventional reflectors are not suitable for this wavelength range.
Work in the field of EUVL (Extreme Ultra-Violet Lithography) was concentrated in the past 1½ decades essentially on the development of molybdenum/silicon—or molybdenum/beryllium multilayers as reflectors for these purposes, specifically mainly for the wavelength range of 11-13 nm. It has been found theoretically that multilayers of this type should have a very high reflectivity of up to 70% in the wavelength range of 11-13 nm. As a result of large worldwide efforts, these high reflectivities could in fact be achieved in the last 1½ decades in experiments—see David Attwood referred to earlier.
For the wavelengths ranges about 7 μm, so far, experimentally a reflectivity of maximally only 20% has been achieved which is insufficient for the lithographic processes for the manufacture of semiconductor structures below the 100 nm mark.
For the optical arrangement of EUVL exposure systems, normally typically six reflectors are used. It is in this connection very important that the individual mirrors have a very high reflectivity. If a system includes for example six reflectors with individual reflectivities of 70% the overall optical transmission of the system is 0.76, that is, only 11.8%. With individual reflectivities of the reflectors of 20% the overall system reflectivity would result in a negligible value of 0.0064%.
For the manufacture of highly integrated semi-conductor structures utilizing the EUVL exposure system a plurality of reflectors are used. It is important that the individual reflectors have an extremely high reflectivity.
The multi-layer reflectors with layers of molybdenum/silicon and molybdenum/beryllium as mentioned earlier are considered to have an acceptable reflectivity in the respective wavelengths of 13-11 nm. However, this reflectivity is really insufficient as shown above, since it is a function of the respective wavelengths, and the light source for the EUVL exposure taking into consideration the increasing density of the semiconductor structures has not been selected yet. The best light sources required for such exposure have not yet been selected; they may not be in the wavelength range mentioned.
It is therefore the object of the present invention to provide a mirror element for the reflection of x-rays, which are suitable for even shorter wavelengths than utilized so far and which, with an essentially normal light incident angle, as it is used at least in connection with conventionally employed reflectors with multiple layers of molybdenum/silicone or molybdenum beryllium or, respectively, have a reflectivity of more than 70%. It should be possible to use the techniques and equipment employed for the manufacture of the conventional mirror elements for the manufacture of such highly integrated semiconductor structures. They should be capable of reflecting light in the wavelength range used herefor, particularly a wave length greater than 6.5 nm; and it should still be possible to manufacture them inexpensively.